What is the speed of sound? Laws of propagation of sound waves

Hydroacoustics (from Greek hydor- water, acousticoc- auditory) - the science of phenomena occurring in the aquatic environment and associated with the propagation, emission and reception of acoustic waves. It includes issues of development and creation of hydroacoustic devices intended for use in the aquatic environment.

History of development

The first measurements of distance through sound were made by Russian researcher Academician Ya. D. Zakharov. On June 30, 1804, he flew in a balloon for scientific purposes and in this flight he used the reflection of sound from the surface of the earth to determine the flight altitude. While in the ball's basket, he shouted loudly into a downward-pointing speaker. After 10 seconds a clearly audible echo came. From this Zakharov concluded that the height of the ball above the ground was approximately 5 x 334 = 1670 m. This method formed the basis of radio and sonar.

Along with the development of theoretical issues, practical studies of the phenomena of sound propagation in the sea were carried out in Russia. Admiral S. O. Makarov in 1881 - 1882 proposed using a device called a fluctometer to transmit information about the speed of currents under water. This marked the beginning of the development of a new branch of science and technology - hydroacoustic telemetry.

Hydrophonic station diagram Baltic plant model 1907: 1 - water pump; 2 - pipeline; 3 - pressure regulator; 4 - electromagnetic hydraulic valve (telegraph valve); 5 - telegraph key; 6 - hydraulic membrane emitter; 7 - side of the ship; 8 - water tank; 9 - sealed microphone

In the 1890s. At the Baltic Shipyard, on the initiative of Captain 2nd Rank M.N. Beklemishev, work began on the development of hydroacoustic communication devices. The first tests of a hydroacoustic emitter for underwater communication were carried out at the end of the 19th century. in the experimental pool in Galernaya Harbor in St. Petersburg. The vibrations it emitted could be clearly heard 7 miles away on the Nevsky floating lighthouse. As a result of research in 1905. created the first hydroacoustic communication device, in which the role of the transmitting device was played by a special underwater siren, controlled by a telegraph key, and the signal receiver was a carbon microphone attached from the inside to the ship's hull. The signals were recorded by a Morse apparatus and by ear. Later, the siren was replaced with a membrane-type emitter. The efficiency of the device, called the hydrophonic station, increased significantly. Sea trials of the new station took place in March 1908. on the Black Sea, where the range of reliable signal reception exceeded 10 km.

The first serial sound-underwater communication stations designed by the Baltic Shipyard in 1909-1910. installed on submarines "Carp", "Gudgeon", "Sterlet", « Mackerel" And " Perch". When installing stations on submarines, in order to reduce interference, the receiver was located in a special fairing, towed behind the stern on a cable rope. The British came to such a decision only during the First World War. Then this idea was forgotten and only at the end of the 1950s it began to be used again in different countries when creating noise-resistant sonar ship stations.

The impetus for the development of hydroacoustics was the First World War. During the war, the Entente countries suffered heavy losses in their merchant and military fleets due to the actions of German submarines. There was a need to find means to combat them. They were soon found. A submarine in a submerged position can be heard by the noise created by the propellers and operating mechanisms. A device that detects noisy objects and determines their location was called a noise direction finder. French physicist P. Langevin in 1915 proposed using a sensitive receiver made of Rochelle salt for the first noise direction-finding station.

Basics of hydroacoustics

Features of the propagation of acoustic waves in water

Components of an echo event.

Beginning of comprehensive and basic research on the propagation of acoustic waves in water was initiated during the Second World War, which was dictated by the need to solve practical problems of navies and, first of all, submarines. Experimental and theoretical work was continued in the post-war years and summarized in a number of monographs. As a result of these works, some features of the propagation of acoustic waves in water were identified and clarified: absorption, attenuation, reflection and refraction.

Absorption of acoustic wave energy in sea ​​water is caused by two processes: internal friction of the medium and dissociation of salts dissolved in it. The first process converts the energy of an acoustic wave into heat, and the second, transforming into chemical energy, removes molecules from an equilibrium state, and they disintegrate into ions. This type of absorption increases sharply with increasing frequency of acoustic vibration. The presence of suspended particles, microorganisms and temperature anomalies in water also leads to attenuation of the acoustic wave in water. As a rule, these losses are small and are included in the total absorption, but sometimes, as, for example, in the case of scattering from the wake of a ship, these losses can amount to up to 90%. The presence of temperature anomalies leads to the fact that the acoustic wave falls into acoustic shadow zones, where it can undergo multiple reflections.

The presence of interfaces between water - air and water - bottom leads to the reflection of an acoustic wave from them, and if in the first case the acoustic wave is completely reflected, then in the second case the reflection coefficient depends on the bottom material: a muddy bottom reflects poorly, sandy and rocky ones reflect well. . At shallow depths, due to multiple reflections of the acoustic wave between the bottom and the surface, an underwater sound channel appears, in which the acoustic wave can propagate over long distances. Changing the speed of sound at different depths leads to bending of sound “rays” - refraction.

Sound refraction (curvature of the sound beam path)

Dispersion and absorption of sound by inhomogeneities of the medium.

Sound wave propagation range

The propagation range of sound waves is complex function radiation frequency, which is uniquely related to the wavelength of the acoustic signal. As is known, high-frequency acoustic signals quickly attenuate due to strong absorption by the aquatic environment. Low-frequency signals, on the contrary, are capable of propagating over long distances in the aquatic environment. Thus, an acoustic signal with a frequency of 50 Hz can propagate in the ocean over distances of thousands of kilometers, while a signal with a frequency of 100 kHz, typical for side-scan sonar, has a propagation range of only 1-2 km. Approximate ranges of modern sonars with different frequency acoustic signal (wavelength) are given in the table:

Areas of use.

Hydroacoustics has received widespread practical use, since it has not yet been created effective system transfers electromagnetic waves under water at any considerable distance, and sound is therefore the only possible means communications underwater. For these purposes, sound frequencies from 300 to 10,000 Hz and ultrasound from 10,000 Hz and above are used. Electrodynamic and piezoelectric emitters and hydrophones are used as emitters and receivers in the audio domain, and piezoelectric and magnetostrictive ones in the ultrasonic domain.

The most significant applications of hydroacoustics:

• To solve military problems;
• Marine navigation;
• Sound communication;
• Fishing exploration;
• Oceanological research;
• Areas of activity for the development of the resources of the ocean floor;
• Using acoustics in the pool (at home or in a synchronized swimming training center)
• Sea animal training.

Notes

Literature and sources of information

LITERATURE:

• V.V. Shuleikin Physics of the sea. - Moscow: “Science”, 1968. - 1090 p.
• I.A. Romanian Basics of hydroacoustics. - Moscow: “Shipbuilding”, 1979 - 105 p.
• Yu.A. Koryakin Hydroacoustic systems. - St. Petersburg: “Science of St. Petersburg and the sea power of Russia”, 2002. - 416 p.

We perceive sounds at a distance from their sources. Usually sound reaches us through the air. Air is an elastic medium that transmits sound.

If the sound transmission medium is removed between the source and the receiver, the sound will not propagate and, therefore, the receiver will not perceive it. Let's demonstrate this experimentally.

Let's place an alarm clock under the bell of the air pump (Fig. 80). As long as there is air in the bell, the sound of the bell can be heard clearly. As the air is pumped out from under the bell, the sound gradually weakens and finally becomes inaudible. Without a transmission medium, the vibrations of the bell plate cannot travel, and the sound does not reach our ear. Let's let air under the bell and hear the ringing again.

Rice. 80. Experiment proving that sound does not propagate in space where there is no material medium

Elastic substances conduct sounds well, such as metals, wood, liquids, and gases.

Let's put a pocket watch on one end of a wooden board, and move to the other end. Putting your ear to the board, you can hear the clock ticking.

Tie a string to a metal spoon. Place the end of the string to your ear. When you hit the spoon, you will hear a strong sound. We will hear an even stronger sound if we replace the string with wire.

Soft and porous bodies are poor conductors of sound. To protect a room from intrusion extraneous sounds, walls, floor and ceiling are laid with layers of sound-absorbing materials. Felt, pressed cork, porous stones, and various synthetic materials (for example, polystyrene foam) made from foamed polymers are used as interlayers. The sound in such layers quickly fades.

Liquids conduct sound well. Fish, for example, are good at hearing footsteps and voices on the shore; this is known to experienced fishermen.

So, sound propagates in any elastic medium - solid, liquid and gaseous, but cannot propagate in space where there is no substance.

The oscillations of the source create an elastic wave of sound frequency in its environment. The wave, reaching the ear, affects the eardrum, causing it to vibrate at a frequency corresponding to the frequency of the sound source. Trembling eardrum transmitted through the ossicular system to the endings of the auditory nerve, irritating them and thereby causing the sensation of sound.

Let us recall that only longitudinal elastic waves can exist in gases and liquids. Sound in the air, for example, is transmitted by longitudinal waves, i.e., alternating condensations and rarefactions of air coming from the sound source.

A sound wave, like any other mechanical waves, does not propagate in space instantly, but at a certain speed. You can verify this, for example, by watching gunfire from afar. First we see fire and smoke, and then after a while we hear the sound of a shot. The smoke appears at the same time the first sound vibration occurs. By measuring the time interval t between the moment the sound appears (the moment the smoke appears) and the moment it reaches the ear, we can determine the speed of sound propagation:

Measurements show that the speed of sound in air at 0 °C and normal atmospheric pressure is 332 m/s.

The higher the temperature, the higher the speed of sound in gases. For example, at 20 °C the speed of sound in air is 343 m/s, at 60 °C - 366 m/s, at 100 °C - 387 m/s. This is explained by the fact that with increasing temperature, the elasticity of gases increases, and the greater the elastic forces that arise in the medium during its deformation, the greater the mobility of particles and the faster vibrations are transmitted from one point to another.

The speed of sound also depends on the properties of the medium in which sound travels. For example, at 0 °C the speed of sound in hydrogen is 1284 m/s, and in carbon dioxide - 259 m/s, since hydrogen molecules are less massive and less inert.

Nowadays, the speed of sound can be measured in any environment.

Molecules in liquids and solids are closer together and interact more strongly than gas molecules. Therefore, the speed of sound in liquid and solid media is greater than in gaseous media.

Since sound is a wave, to determine the speed of sound, in addition to the formula V = s/t, you can use the formulas you know: V = λ/T and V = vλ. When solving problems, the speed of sound in air is usually considered to be 340 m/s.

Questions

1. What is the purpose of the experiment depicted in Figure 80? Describe how this experiment is carried out and what conclusion follows from it.
2. Can sound travel in gases, liquids, and solids? Support your answers with examples.
3. Which bodies conduct sound better - elastic or porous? Give examples of elastic and porous bodies.
4. What kind of wave - longitudinal or transverse - is sound propagating in the air? in water?
5. Give an example showing that a sound wave does not travel instantly, but at a certain speed.

Exercise 30

1. Could the sound of a huge explosion on the Moon be heard on Earth? Justify your answer.
2. If you tie one half of a soap dish to each end of the thread, then using such a telephone you can even talk in a whisper while in different rooms. Explain the phenomenon.
3. Determine the speed of sound in water if a source oscillating with a period of 0.002 s excites waves in water with a length of 2.9 m.
4. Determine the wavelength of a sound wave with a frequency of 725 Hz in air, in water and in glass.
5. One end of a long metal pipe was struck once with a hammer. Will the sound from the impact spread to the second end of the pipe through the metal; through the air inside the pipe? How many blows will a person standing at the other end of the pipe hear?
6. Observer standing near a straight line railway, saw steam above the whistle of a steam locomotive going in the distance. 2 seconds after the steam appeared, he heard the sound of a whistle, and after 34 seconds the locomotive passed by the observer. Determine the speed of the locomotive.

Sound waves can propagate in various media - liquid, solid and gaseous. Waves cannot form only in vacuum. The denser the medium, the higher the speed of sound propagation in it. In water, the speed achieved by sound waves is more than four times higher than the speed of their propagation in air.

Here is an explanation of this phenomenon from a physics point of view:



Sound travels faster in an elastic medium. The higher the density of this medium, the more favorable it is for the propagation of sound vibrations. The speed of sound in water reaches 1500 meters per second, and in air - only 330-340 m/s; the speed also depends on temperature.

For comparison, the speed of sound in metals is 5000 meters per second.

Sound waves do not propagate only in airless space; in liquid, gaseous, and also solid media, sound waves propagate calmly.

The speed of propagation of sound waves in a straight line depends on the density of the medium, which is higher density environment, the stronger the speed of wave propagation.

The density of water is much higher than the density of air, therefore the speed of the sound wave in water is higher.

As an argument, Volodya, you cite main reason. Yes. Because water is a less compressible medium than gas. And a solid is less compressible (during wave propagation) than a liquid. Water at great depths conducts sound faster than at the surface; it is more compressed there. There is an inversely proportional relationship between the speed of sound and the density of the medium. In other words, the less compressible the wave propagation medium is, the faster the wave moves.

I'll give you a rough analogy. When the train starts moving, a sort of clanging wave runs through the train and the last car starts moving some time after the locomotive began to move. The same thing, but in reverse order, happens during a stop. And all because the medium is compressible, there is a certain gap between the cars, which plays the role of the compressibility of the medium. If at the moment of starting (stopping) the entire train is tense or compressed (for example, it is not on a horizontal platform), then the last car will start (stop) almost simultaneously with the locomotive. The medium is non-compressible and the wave propagates much faster.

Sound is waves that travel through any substance. Air is a rarefied substance, and water is a much denser substance than air. Therefore, sound waves travel faster in water than in air.

Sound waves are divided into longitudinal and transverse. The speed of sound propagation depends on the density of the medium and can vary over a fairly wide range. In water and in a gaseous medium, where density fluctuations are not significant, acoustic waves propagate longitudinally, that is, the direction of vibration of the particles of the medium coincides with the direction of movement of the wave. In dense (solid) bodies, in addition to longitudinal movements, elastic shear deformations also occur, which causes the occurrence of transverse (shear) waves; Therefore, the particles oscillate perpendicular to the direction of propagation of the wave. In addition to the direction of wave propagation, acoustic resistance and pressure of the medium also play a role. In addition, the speed of sound also depends on such factors as the compressibility of substances.

It is under water that sound travels faster than in air, five times faster.

Even whales can hear each other at a distance of 5 kilometers.

So why does sound travel faster underwater? It's all about density!

The density of water is greater than that of air, but also less than that of metal. Accordingly, sounds will be transmitted differently.

But sound waves can propagate even in elastic media, for example, if you put your ear to the ground, you can hear the sound of footsteps, the clatter of hooves, a car driving and much more.

Sound is mechanical vibrations transmitted in any medium and perceived by the senses. Because of physical properties different environments, the speed of propagation of sound vibrations is different. The denser the medium, the higher the speed of sound transmission. Answer to the task: Sound waves in water travel faster than in air, for the reason that water has a higher density.

IN clean water the speed of sound is 1500 meters per second, and increases in warmer and more salty water. Water is denser than air, so sound travels faster. In addition, a person perceives sound underwater through the bones of the skull, and the sound is perceived by both ears, which makes it seem as if sounds are coming from all sides.

>>Physics: Sound in various environments

For sound to propagate, an elastic medium is required. In a vacuum, sound waves cannot propagate, since there is nothing there to vibrate. This can be verified by simple experience. If we place an electric bell under a glass bell, then as the air is pumped out from under the bell, we will find that the sound from the bell will become weaker and weaker until it stops completely.

Sound in gases. It is known that during a thunderstorm we first see a flash of lightning and only after some time we hear the rumble of thunder (Fig. 52). This delay occurs because the speed of sound in air is much less than the speed of light coming from lightning.

The speed of sound in air was first measured in 1636 by the French scientist M. Mersenne. At a temperature of 20 °C it is equal to 343 m/s, i.e. 1235 km/h. Note that it is to this value that the speed of a bullet fired from a Kalashnikov machine gun (PK) decreases at a distance of 800 m. The initial speed of the bullet is 825 m/s, which significantly exceeds the speed of sound in air. Therefore, a person who hears the sound of a shot or the whistle of a bullet need not worry: this bullet has already passed him. The bullet outruns the sound of the shot and reaches its victim before the sound arrives.

The speed of sound depends on the temperature of the medium: with increasing air temperature it increases, and with decreasing air temperature it decreases. At 0 °C, the speed of sound in air is 331 m/s.

In different gases, sound travels with at different speeds. The greater the mass of gas molecules, the lower the speed of sound in it. Thus, at a temperature of 0 °C, the speed of sound in hydrogen is 1284 m/s, in helium - 965 m/s, and in oxygen - 316 m/s.

Sound in liquids. The speed of sound in liquids is usually greater than the speed of sound in gases. The speed of sound in water was first measured in 1826 by J. Colladon and J. Sturm. They carried out their experiments on Lake Geneva in Switzerland (Fig. 53). On one boat they set fire to gunpowder and at the same time struck a bell lowered into the water. The sound of this bell, using a special horn, also lowered into the water, was captured on another boat, which was located at a distance of 14 km from the first. Based on the time interval between the flash of light and the arrival of the sound signal, the speed of sound in water was determined. At a temperature of 8 °C it turned out to be approximately 1440 m/s.

On the border between two different environments Some of the sound wave is reflected, and some travels further. When sound passes from air into water, 99.9% of the sound energy is reflected back, but the pressure in the sound wave transmitted into the water is almost 2 times greater. The hearing system of fish reacts precisely to this. Therefore, for example, screams and noises above the surface of the water are the right way scare away sea creatures. A person who finds himself under water will not be deafened by these screams: when immersed in water, air “plugs” will remain in his ears, which will save him from sound overload.

When sound passes from water to air, 99.9% of the energy is reflected again. But if during the transition from air to water the sound pressure increased, now, on the contrary, it sharply decreases. It is for this reason, for example, that the sound that occurs under water when one stone hits another does not reach a person in the air.

This behavior of sound at the boundary between water and air gave our ancestors the basis to consider the underwater world a “world of silence.” Hence the expression: “Mute as a fish.” However, Leonardo da Vinci also suggested listening to underwater sounds by putting your ear to an oar lowered into the water. Using this method, you can make sure that the fish are actually quite talkative.

Sound in solids . The speed of sound in solids is greater than in liquids and gases. If you put your ear to the rail, you will hear two sounds after hitting the other end of the rail. One of them will reach your ear by rail, the other by air.

The earth has good sound conductivity. Therefore, in the old days, during a siege, “listeners” were placed in the fortress walls, who, by the sound transmitted by the earth, could determine whether the enemy was digging into the walls or not. Putting their ears to the ground, they also monitored the approach of enemy cavalry.

Solids conduct sound well. Thanks to this, people who have lost their hearing are sometimes able to dance to music that reaches them. auditory nerves not through the air and the outer ear, but through the floor and bones.

1. Why during a thunderstorm do we first see lightning and only then hear thunder? 2. What does the speed of sound in gases depend on? 3. Why doesn’t a person standing on the river bank hear sounds arising under water? 4. Why were the “hearers” who in ancient times monitored the enemy’s excavation work often blind people?

Experimental task . Place a board (or long wooden ruler) on one end wrist watch, put your ear to its other end. What do you hear? Explain the phenomenon.

S.V. Gromov, N.A. Rodina, Physics 8th grade

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Over long distances, sound energy travels only along gentle rays that do not touch the ocean floor along the entire path. In this case, the limitation imposed by the environment on the range of sound propagation is its absorption in sea water. The main mechanism of absorption is associated with relaxation processes accompanying the disturbance by an acoustic wave of the thermodynamic equilibrium between the ions and molecules of salts dissolved in water. It should be noted that the main role in absorption in a wide range of sound frequencies belongs to the magnesium sulfur salt MgSO4, although in percentage terms its content in sea water is very small - almost 10 times less than, for example, NaCl rock salt, which nevertheless does not play a role any significant role in sound absorption.

Absorption in sea water, generally speaking, is greater the higher the sound frequency. At frequencies from 3-5 to at least 100 kHz, where the above mechanism dominates, absorption is proportional to frequency to the power of approximately 3/2. At lower frequencies, a new absorption mechanism is activated (possibly due to the presence of boron salts in water), which becomes especially noticeable in the range of hundreds of hertz; here the level of absorption is anomalously high and falls significantly more slowly with decreasing frequency.

To more clearly imagine the quantitative characteristics of absorption in sea water, we note that due to this effect, sound with a frequency of 100 Hz is attenuated 10 times over a path of 10 thousand km, and with a frequency of 10 kHz - at a distance of only 10 km (Figure 2). Thus, only low-frequency sound waves can be used for long-distance underwater communication, long-range detection of underwater obstacles, etc.

Figure 2 - Distances at which sounds of different frequencies attenuate 10 times when propagating in sea water.

In the region of audible sounds for the frequency range 20-2000 Hz, the propagation range of medium-intensity sounds under water reaches 15-20 km, and in the ultrasound region - 3-5 km.

Based on the sound attenuation values ​​observed in laboratory conditions in small volumes of water, one would expect significantly greater ranges. However, in natural conditions In addition to attenuation caused by the properties of water itself (the so-called viscous attenuation), its scattering and absorption by various inhomogeneities of the medium also affect it.

Refraction of sound, or curvature of the path of a sound beam, is caused by heterogeneity in the properties of water, mainly vertically, due to three main reasons: changes in hydrostatic pressure with depth, changes in salinity and changes in temperature due to unequal heating of the water mass by the sun's rays. As a result of the combined effect of these reasons, the speed of sound propagation, which is about 1450 m/sec for fresh water and about 1500 m/sec for sea water, changes with depth, and the law of change depends on the time of year, time of day, depth of the reservoir and a number of other reasons. . Sound rays emerging from the source at a certain angle to the horizon are bent, and the direction of the bend depends on the distribution of sound speeds in the medium. In summer, when the upper layers are warmer than the lower ones, the rays bend downwards and are mostly reflected from the bottom, losing a significant share of their energy. On the contrary, in winter, when the lower layers of water maintain their temperature, while the upper layers cool, the rays bend upward and undergo multiple reflections from the surface of the water, during which much less energy is lost. Therefore, in winter the range of sound propagation is greater than in summer. Due to refraction, so-called dead zones, i.e. areas located close to the source in which there is no audibility.

The presence of refraction, however, can lead to an increase in the range of sound propagation - the phenomenon of ultra-long-range propagation of sounds under water. At some depth below the surface of the water there is a layer in which sound travels at the lowest speed; Above this depth, the speed of sound increases due to an increase in temperature, and below this depth, due to an increase in hydrostatic pressure with depth. This layer is a kind of underwater sound channel. A beam that has deviated from the axis of the channel up or down, due to refraction, always tends to fall back into it. If you place the source and receiver of sound in this layer, then even sounds of medium intensity (for example, explosions of small charges of 1-2 kg) can be recorded at distances of hundreds and thousands of km. A significant increase in the range of sound propagation in the presence of an underwater sound channel can be observed when the sound source and receiver are located not necessarily near the channel axis, but, for example, near the surface. In this case, the rays, refracting downward, enter deep-sea layers, where they are deflected upward and exit again to the surface at a distance of several tens of kilometers from the source. Next, the pattern of ray propagation is repeated and as a result a sequence of so-called rays is formed. secondary illuminated zones, which are usually traced to distances of several hundred km.

The propagation of high-frequency sounds, in particular ultrasounds, when the wavelengths are very small, is influenced by small inhomogeneities usually found in natural bodies of water: microorganisms, gas bubbles, etc. These inhomogeneities act in two ways: they absorb and scatter the energy of sound waves. As a result, as the frequency of sound vibrations increases, the range of their propagation decreases. This effect is especially noticeable in the surface layer of water, where there are most inhomogeneities. The scattering of sound by inhomogeneities, as well as uneven surfaces of water and the bottom, causes the phenomenon of underwater reverberation, which accompanies the sending of a sound impulse: sound waves, reflecting from a set of inhomogeneities and merging, give rise to a prolongation of the sound impulse, which continues after its end, similar to the reverberation observed in enclosed spaces. Underwater reverberation is a fairly significant interference for a number of practical applications of hydroacoustics, in particular for sonar.

The range of propagation of underwater sounds is also limited by the so-called. the sea's own noises, which have a dual origin. Some of the noise comes from the impact of waves on the surface of the water, from the sea surf, from the noise of rolling pebbles, etc. The other part is related to marine fauna; This includes sounds made by fish and other marine animals.